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You have to clear Immigration first. If you have international flight, be at the airport 2.5 - 3 hours prior to your flight's take off. That gives you approximately 3 hours to go from Airport T3 Terminal to India Gate and come back. It takes approximately 1 hour to and fro if you travel via metro which is better for commute in my opinion. Remaining 2 hours should be enough to get a decent meal and enjoy the sightseeing.
